Organic light-emitting display (OLED) device possesses advantages, such as, self-illumination, quick response, wide viewing-angle, high brightness, vivid colors and small weight over liquid crystal display (LCD) device, and hence has been considered as a next-generation technology. Self-illuminous unit such as OLED unit in the OLED display device are mainly consisted of an anode, a light-emitting functional layer and a cathode which are disposed sequentially in a direction away from a substrate. Depending on the difference in illumination direction, the OLED unit may be classified into bottom-emission type (i.e., emitting light downwardly with respect to the substrate) and top-emission type (i.e., emitting light upwardly with respect to the substrate). The top-emission mode may obtain a higher aperture ratio, and thus, existing OLED display devices mostly adopt top-emission type OLED units.
Since the cathode is usually made of pure metal and/or alloy material having low work function and is relatively poor in transmittance, in order to decrease an influence to an overall light extracting ratio of the OLED unit (especially for top-emission type OLED unit), the cathode normally has a very small thickness. Based on a square resistance expression Rs=ρ/t of an electrode (ρ is a resistivity of the electrode, and t is a thickness of the electrode), it can be seen that the square resistance of the cathode is increased with the decrease of the thickness thereof, accompanying with a severe voltage drop (IR drop, i.e., a potential difference between two ends of the electrode) of the OLED unit, which results in the voltage drop at a light-emitting surface of the OLED unit increasingly obvious with an increased distance from a power supply, that is, a driving transistor connected to the anode layer, thereby leading to significantly uneven illumination of the OLED unit.
As illustrated in FIG. 1, the cathode 1 of the OLED unit (including the cathode 1, the anode 2 and the light-emitting functional layer 3 located between the cathode 1 and the anode 2) is required to be connected to a print circuit board (PCB) 4. Thus it needs to dispose a via hole 6 in a non-display area (NDA) at a periphery of a display area (DA) to connect the cathode 1 to the PCB 4 through a conductive lead 5. Therefore, the via hole structure in the NDA occupies a relatively larger area of the periphery of the display screen, which significantly influences the area of the display screen of the entire OLED display device.